Traditionally, various methods have been proposed to produce a water-soluble cationic polymer used for flocculants, antistatic agents, retention aids for paper manufacturing and other purposes, including polymerization in aqueous solution (e.g., Japanese Patent Laid-Open No. 39379/1975), water-in-oil emulsion polymerization (e.g., Japanese Patent Laid-Open No. 102388/1979) and suspension polymerization in hydrophobic solvent (e.g., Japanese Patent Laid-Open Nos. 69196/1979 and 66911/1980).
At present, homopolymers of cationic methacrylate monomer or cationic acrylate monomer, or copolymers of said monomer and another monomer copolymerizable with said monomer (e.g., acrylamide) are produced by polymerization in aqueous solution. Generally, many of them contain a high ratio of cationic methacrylate monomer. This is because those containing a high ratio (not less than 60%) of cationic acrylate monomer are very difficult to produce. In other words, this is due to the fact that cationic acrylate monomer is more sensitive to hydrolysis than cationic methacrylate monomer, and the hygroscopicity of cationic acrylate polymer is higher than that of cationic methacrylate polymer. Thus, when producing a polymer mainly based on cationic acrylate monomer, it is necessary to maximize the concentration of said monomer to suppress the hydrolysis of said monomer, but when the monomer concentration is increased, polymerization temperature becomes difficult to control so that it becomes too high, which results in hydrolysis. It is not desirable to conduct polymerization at the cost of hydrolysis of cationic acrylate monomer, because the expensive cationic monomer is wasted. Other problems have also been pointed out. For example, the polymer obtained adheres to the inside wall of drier during drying because it is highly hygroscopic, which in turn lowers the drying efficiency. In addition, the polymer is difficult to pulverize because once-pulverized particles mutually adhere again when pulverizing. This tendency becomes more marked as the ratio of cationic acrylate monomer increases.
Water-in-oil emulsion polymerization has the disadvantage that the use of the product which is in an emulsion form is very dangerous because the product contains a large amount of ignitable organic solvent, and further the precious organic solvent is wasted.
In suspension polymerization, the use of a sorbitan fatty acid ester, such as sorbitan monooleate or ethyl cellulose, as a surfactant is known. However, when sorbitan monooleate is used for suspension polymerization, the particle size of the obtained polymer is very fine, at 10 to 100 .mu.m in diameter. Therefore, it is necessary to take a countermeasure against dust formation in handling the powder, and the poor fluidity makes its handling very difficult. In addition, it is very difficult to dissolve in water because formation of powder lump is likely to occur due to the smallness of the particle size.
When ethyl cellulose is used for suspension polymerization, the amount of ethyl cellulose used increases because of the inferior polymerization stability, which results in an increase in insoluble substances when the polymer is dissolved in water. Also, adhesion to the drier is remarkable, which results in entire or partial aggregation of the product. This method is therefore undesirable.
As stated above, various methods have been proposed to produce a water-soluble cationic polymer, but all have various drawbacks as described above. Thus, there has been demand for a new method free of these problems by which a polymer with a high degree of cationization and a high viscosity when dissolved in water is industrially advantageously produced, but there has been found no useful method.